1. Field of the Invention
This invention generally pertains to a cylinder block structure of an engine and more particularly to a cylinder block structure featuring improved sealing with a cylinder head.
2. Description of the Related Art
The seal between a cylinder block and cylinder head of an engine is maintained by using head bolts to bolt down the cylinder head on the cylinder block with an intervening gasket. More specifically, the head bolts are inserted through the cylinder head and tightened down in the female threads of head bolt holes in the cylinder block such that axial tension develops in the female threads and is transmitted to the top deck of the cylinder block, whereupon the top face (sealing face) of the cylinder block is pressed against the cylinder head to form a seal.
In the interest of improving the seal, it is desirable that the axial tension which develops in the female threads due to tightening down the head bolts acts more or less uniformly over the entire sealing face. Conventionally, cast iron has been used as the cylinder block material in engines. Cast iron exhibits a large Young's modulus, wherefore the axial tension developing in the female threads is transmitted not only to the vicinity about the head bolt holes, but to the entire seal face, inclusive of the portions between neighboring head bolt holes, so that adequate sealing can be secured.
Due to the demand for lighter weight in vehicles in recent years, however, cylinder block material is being changed from cast iron to aluminum. Aluminum exhibits a smaller Young's modulus than does cast iron. For this reason, the axial tension developed in the female threads is concentrated about the periphery of the head bolt holes, and is not transmitted to the entire sealing face, resulting in uneven sealing forces. The sealing forces are particularly inadequate in the places lying intermediately between neighboring head bolt holes, wherefore there is a possibility of gas escaping from the cylinders through these places.
One possible approach for resolving this drawback is to increase the number of cylinder bolts and thereby narrow the intervals between head bolt holes. When the number of bolts is increased, however, restrictions are thereby placed on the numbers and shapes of intake and exhaust ports formed in the cylinder head, resulting in diminished design freedom.